An example of a conventionally known fuel cell system is the one disclosed in JP-A 2009-4180 which controls its peripheral devices based on hydrogen concentration. This fuel cell system has a hydrogen concentration sensor located on the downstream side of the anode exit of its fuel cell. The hydrogen concentration sensor detects the hydrogen concentration of anode off-gas discharged from the anode exit of the fuel cell.
The above fuel cell system is designed to control the anode exit valve of the fuel cell based on the output of the hydrogen concentration sensor. Specifically, this anode exit valve is controlled such that the fuel cell receives hydrogen the amount of which matches the hydrogen consumption amount of the fuel cell. According to the above Patent Document 1, such control of the abode exit valve leads to a reduction in the hydrogen concentration of the exhaust gas discharged through the anode exit valve.
It is also known that there exist techniques for examining the states of a fuel cell (e.g., the amount of water contained in the fuel cell) by measuring its impedance. As a method of fuel-cell impedance measurement, the AC (alternating-current) impedance method is known.
A measurement result obtained under the AC impedance method is often graphically represented in a complex plane. A diagram of this graphical representation is commonly referred to as a Cole-Cole plot. A Cole-Cole plot is obtained by measuring a plurality of impedance values according to the AC impedance method while varying the voltage frequency and then plotting the values in a complex plane. As is known, a curve passing through impedance points in a Cole-Cole plot (i.e., a trajectory of impedance values) takes the form of a semicircle. Hereinafter, this semicircle is also referred to as an “impedance semicircle.”
JP-A 2008-8750, though not related to fuel cells, discloses a technique for measuring the amount of hydrogen peroxide based on impedance semicircles.
Naoki Ito, et al. disclosed fuel-cell analysis results based on impedance semicircles (2008. Electrochemical analysis of hydrogen membrane fuel cells. Journal of Power Sources, 185, pp. 922-926.) (Non-Patent Document 1). This document discloses analysis of the electrochemical characteristics of hydrogen membrane fuel cells (HMFCs). FIGS. 1(b) and (c) of this document are diagrams related to anode hydrogen concentrations, cathode oxygen concentrations, and impedance semicircles.